It's not long young (5)
I believe the answer is:
tooth
'young' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'it's not long' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'not' becomes 'o' (I've seen this in other clues).
'long' becomes 'oth' (I can't justify this - if you can you should give a lot more credence to this answer).
't'+'o'+'oth'='TOOTH'
Can you help me to learn more?
(Other definitions for tooth that I've seen before include "Bonelike structure" , "Canine, for example" , "Taste" , "One needed to chew" , "Incisor, e.g." .)